The most widely used laser spot tracker techniques employ a conventional quadrant array of photodetectors located in or near the focal plane of a moderately fast optical system. The exact configuration of such systems is usually dictated by trade-offs between signal/noise considerations and field-of-view. For example, a wide field-of-view (FOV) is desirable because of ease of target acquisition and for maintainance of track-lock on rapidly maneuvering targets. A wide FOV however implies larger detector elements. Large detector elements are undesirable from the standpoints of single/noise and the fact that as the detector elements increase in size relative to the image spot size, there is a tendency toward a bang-bang or limit-cycle response. A complicating factor is the effect of atmospheric turbulence on the irradiance distribution at the tracker aperture. If the image is defocused to provide a larger spot size, then entrance aperture irradiance spatial and temporal fluctuations will produce similar fluctuations in the defocused image, with resultant spurious error signals.
A laser spot tracker which used a focused spot, to avoid the effects of scintillation, thus permitting the use of smaller detectors for an improved signal/noise ratio, yet had a wide field-of-view, would be desirable and useful.